Electric heating pads have been used for a long time for healing or soothing bruised areas of the body. Electric heating pads are generally characterized by the fact that they are flexible and therefore can be adapted to parts of the body that need to be heated. The traditional heating pad consists of a rope heater, namely, a heating element where a very fine electric conductor wire is wound around a core which is typically made of glass fibers, and which is manufactured in long-length and cut to the required length and then stitched in a meandering manner onto a substrate made of felt or similar material which supports it in its position. The substructure is subsequently covered by a couple of sheets of vinyl, one on each side of the felt and the vinyl sheets are then adhered together at their peripheral edges to provide a water tight enclosure.
Rope heaters have evolved over the years to where now-a-days, the rope heater is a structure that consists of the aforementioned fiberglass core, surrounded by fine resistance wire. However, the entire heater is subsequently covered by an insulating layer of PVC to prevent contact with the electrically live wire. The rope heater, in its early form or in its present form, is of a diameter that approaches one eighth ('1/8") of an inch and is stitched in a meandering pattern to the supporting felt pad. However, it can be felt from the outside as ridges that are uncomfortable for the user and due to the fact that they are spaced apart between one (1) and one and a half (11/2) inches. The typical heating pad therefore will exhibit hot areas where the rope heater is located, and cool areas in between two adjacent heater turns. By contrast, the heating pad of the present invention, incorporating the electrically conductive sheet, would offer an almost continuous and uninterrupted source of heat over the entire surface of the pad alleviating the problem of the non-uniform heat generation as well as the problem of the ripples, caused by the rope heater in the old designs.
Attempts have been made in the past to use an impregnated board as a flexible heater, as U.S. Pat. No. 3,657,516 to Fujihara teaches the use of an impregnated pad as a flexible source of heat. Also, U.S. Pat. No. 4,697,066 to Glucksman, teaches a flexible heater in conjunction with a curling iron as do U.S. Pat. No. 3,099,540 to Eisler, U.S. Pat. No. 3,266,055 to Balder et al., and U.S. Pat. No. 4,485,297 to Grise et al. The latter four patents reference a printed or a diecut heater in contrast with the impregnated carbon electrically conductive sheet as in the present invention. Of interest will also be U.S. Pat. No. 4,710,612 to Lin et al., as well as U.S. Pat. No. 2,473,183 to Watson. One may also mention U.S. Pat. No. 4,791,276 to Midgley et al. which teaches a somewhat different application of a heater in an elongated device meant for thawing pipes in the winter and preventing freezing of gutters, driveways, etc.
The heating pad, according to the present invention, differs from all of the above references insofar as the electrically conductive impregnated sheet is perforated in predetermined locations and that the outer envelope, made of two PVC sheets which are adhered to each other through those perforations. By doing this, one is able to anchor the electrically conductive impregnated sheet in place so as to avoid it from folding over on itself or its shifting within the envelope, which is a problem that would normally be encountered with such large sheets, resulting in over-heating as well as electrically "shorting" of the electrically conductive sheet on to itself. A further advantage associated with the perforation of the pad is that, by running elongated slots along the conductive path of the pad, one is able to isolate certain resistive portions from the main heater and use these resistive portions in order to control the temperature of the heating pad as will be discussed later on.